Light-sensitive aluminum plate for lithography



United States Patent 3,471,289 LIGHT-SENSITIVE ALUMINUM PLATE FOR LITHOGRAPHY Clifford E. Herrick, Jr., Concord, Mass., assignor, by

mesne assignments, to GAF Corporation, a corporation of Delaware No Drawing. Continuation of application Ser. No. 221,876, Sept. 6,.1962. This application Mar. 21, 1968, Ser. No. 715,104

Int. Cl. G03f 7/10; Clld 9/10 US. C]. 96-33 16 Claims ABSTRACT OF THE DISCLOSURE Light-sensitive plate having a mechanically grained surface of metallic aluminum or aluminum oxide formed by exposure to air, coated with a mixture of an alkaliresistant, water-insoluble, solvent-soluble adhesive polyvinyl acetate or polymethacrylate resin and a light-sensitive water-insoluble orthodiazo oxide derivative of an aromatic sulfonamide having a high molecular weight alicyclic amido nitrogen substituent, and a process for converting the same to a lithographic plate by imagewise exposure to light and development with an aqueous alkaline synthetic detergent solution containing a reagent adapted to convert the aluminum to a hydrophilic film.

This application is a continuation of Ser. No. 221,876, filed Sept. 6, 1962, and now abandoned.

This invention relates to a method for producing a positive printing lithographic plate from a positive original.

Aluminum metal positive presensitized plates 'based on the use of diazo oxide sensitizers have been marketed in this country for a number of years. As shown by Sus (Annalen der Chemie, 556, 65 if. (1944), these lightsensitive, water-insoluble orthoquinone substances suffer a ring collapse on exposure to light, generating a S-membered ring with an attached ketene group side chain which, in the presence of base, yields the salt of the corresponding acid, thereby conferring a degree of water solubility on the photoproduct. This finding was exploited in the making of lithographic plates, first in French patent 904,255 and later in a number of German patents certain of which have matured into corresponding US. patents, including 2,766,118; 2,767,092; 2,859,112; 3,046,131 and 3,046,110-124 inclusive. These patents disclose a wide variety of orthoq-uinone diazides as useful for the production of positive lithographic images. Weakly alkaline soluitions are prescribed as a general recipe for developing these layers (mono, di, and trisodium phosphate, sodium hydroxide, sodium bicarbonate, sodium carbonate and mixtures of certain of these materials comprising the entire list of substances actually employed in the examples in the subject patents). A Wide variety of substrates are disclosed as being useful, although the patents are frequently vague as to the actual processes employed in preparing the aluminum surfaces used in the various examples.

In a typical example, a surface oxidized aluminum plate coated with a diazo oxide and a minor proportion of an alkali-soluble resin is exposed to light, developed with a weakly alkaline solution and then finished by going over the plate with dilute phosphoric acid sometimes followed by gum or in some cases by an aqueous solution comprising ammonium dihydrogen phosphate, glycerine and phosphoric acid or alternatively dextrin, phosphoric acid and formaldehyde in water solution. In other cases merely gum or acid gum is employed. In a variant of this process, the image may first be inked with a greasy ink and then developed with a weakly alkaline 3,471,289 Patented Oct. 7, 1969 'ice solution. In general, no fundamental change in the nonprinting areas surface over that present in the image areas is disclosed. In some cases, e.g., US. Patent 3,046,124, it is noted that the nonprinting areas comprise the base material.

A somewhat different approach to the use of diazo oxides for making printing plates is described in US. Patent 2,772,972 in which a mixture of an alkali-resistant resin and a diazo oxide is used on a resinous plate surface which is fundamentally altered in its surface characteristics in the nonimage portions by the action of a developer comprising an alkanolamine. This process is advantageous since it allows areas to be water repellent and extremely tightly bonded to the non-light-struck sensitizer composition, while the non-image areas through the action of the alkanolamine are fundamentally changed in character and become very hydrophilic.

While the commercial aluminum presensitized plate products have found a market in the smaller and medium sizes, certain shortcomings have become evident in these materials as currently marketed. These shortcomings appear to arise from the necessity of meeting the stringent requirements in shelf life and shipability which a presensitized plate encounters. A large fraction of the cost of a presensitized plate resides in the cost of the metal which is usually aluminum. In order to protect this investment against the ravages of time and transportation, certain compromises in performance are evident. For instance, the preference of domestic lithographers for a fully grained plate surface is well known. Unsensitized grained surfaces require considerably more care in packaging and shipping to avoid damage to the grain peaks than do unsensitized smooth plates. This is especially true if the sharp mechanical grain preferred by a great many printers is involved. In a presensitized plate, these problems are considerably magnified since the relatively soft sensitizer layer can readily be abraded away by slight plate to plate contact during shipping. For this reason many domestic manufacturers of grained presensitized plates (both positive and negative) employ minimum and relatively smooth grain which, in my opinion offers little advantage over smooth surfaces such as that described in US. Patent 2,714,066.

Further, the preferred presensitizing operation on positive plates is carried out in web form, that is to say a continuous coil of metal is run intact through a machine and coated with the sensitizing composition.

In the larger plate sizes it is presently impossible to secure sufiiciently flat sheet in coil form. In these large plate sizes the metal must be thicker, e.g., up to 0.025 inch which is impractical in a coil form. Further, if an attempt is made to fully mechanically grain a plate in web form, a distinct curling tendency is parted to the sheet. While this curl can readily be removed by two decurling passes through decurling rolls normal and transverse to the graining direction, such decurling procedures necessitate the prior conversion of the metal coil to sheet form. Presensitizing in sheet form does not presently appear practical in these large sizes. Thus, in addition to shipping problems posed by full mechanical grain, avoiding curl is an additional reason for the type of grain which appears on the plates presently being marketed as positive presensitized plates.

The aluminum plates of the prior art appear to be coated on at least a partly desensitized aluminum surface,that is to say, an aluminum surface which is to a de gree water-receptive and oil-rejecting per se, at least when compared to an active surface of the type to be described. On the other hand, the prior art plates are not coated over fully densensitized aluminum surfaces since such surfaces would tend to exhibit poor adhesion and short press life.

In summary, the prior art surfaces appear to represent a compromise between securing a moderate degree of adhesion to the sensitizing composition and a sufficient degree of water receptivity to be useful as printing plates.

A general object of this invention is to sensitize a metal surface treated to secure good adhesion and water repellency in the image areas and to convert the nonimage areas of such a surface to a highly hydrophilic, ink-repellent condition without affecting the oleophilic character of the surface in the image portions.

It is a further object of this invention to produce a positive working metal plate having a sharp mechanical grain of the type preferred by a majority of the trade.

Another object of this invention is to make it possible to produce sharp-grained, thick, fiat positive plates in any size to meet the need of printers having large presses.

A further object of this invention is to provide a process making possible the use of sharp, fully grained aluminum surfaces having great affinity for the sensitizing components.

Another object of this invention is to furnish a process yielding nonprinting areas which exhibit a high degree of desensitization so that while the image areas contain areas resembling deepetch image areas, the nonimage areas are entirely different and show a very high degree of water afiinity and oil rejection.

It is a further object of this invention to make it pos sible to employ sensitizing compositions and plate surfaces which are so adapted that the sensitizing composition may be applied by hand with a high degree of uniformity, thus avoiding shipping difliculty mentioned earlier (since during transportation, the sensitizer can be absent) and avoiding any performance comprises related to the necessity for obtaining long shelflife.

Another object ofthis invention is to make it possible to employ sensitizing compositions which contain a substantial porportion of an alkali-resistant adhesive resin of the deep-etch lacquer type in the sensitizing layer, such components affording advantages in both printing speed and run length.

The invention comprises broadly using as a base an aluminum sheet having at least one active, mechanically grained surface. A sensitizing composition is applied which comprises essentially a mixture of alkali-resistant, water-insoluble, solvent-soluble adhesive polyvinyl resin and a light-sensitive, water-insoluble diazo oxide of an aromatic sulfonamide in which the =N and always occupy adjacent positions in the same ring and in which the amido nitrogen is substituted by an alicyclic radical of high molecular weight. The coated plate is exposed to a positive original. The light-struck sensitizer composition is removed by application of a developing solution comprising an aqueous, alkaline solution of synthetic organic detergent and a material characterized as being reactive in alkaline solution with aluminum to produce a hydrophilic film.

The term active as applied to the aluminum surface is used to denote a surface which is essentially aluminum or the aluminum oxides such as are formed by exposure to air and free of hydrophilic films. Preparation of such a surface normally requires careful cleaning and washing and, in some cases, treating with cold concentrated nitric acid to remove smut.

The diazo oxides useful in this invention are those described in U.S. Patent 2,797,213. The alkali-resistant polyvinyl resins useful in this invention are preferably of low molecular weight and include polyvinyl acetate and polymethacrylate.

The sensitizing compositions are preferably applied in a solvent boiling at 150 C. or higher. It is further desirable that the solvent be insensitive to water, since the plates may be used without intensive drying very shortly after coating and thus may retain appreciable quantities of solvent. Under such circumstances, if chemicals are used which have great solvent power and are water-sensitive to a degree, the image areas are prone to be watersensitive and occasionally blind. On the other hand, by the use of relatively high boiling solvents showing little or no affinity for water, extremely smooth layers can be laid down by a hand operation. Cyclohexanone is typical of a preferred solvent. The diazo oxides of this invention are soluble in most common organic solvents excluding saturated hydrocarbons, aliphatic and cyclic.

An important feature of my invention is the type of developer composition employed. The developer composition enables one to alter radically the surface characteristics of the nonprinting portions of the plate surface. As noted earlier, an active mechanically grained aluminum surface tends to bond tightly to many substances, including the photoproduct of the light-sensitive layer of this invention. Such active surfaces in combination with the alkali-resistant resin containing sensitizer compositions of this invention exhibit tenacious mutual adhesion and an astonishingly long press life.

Preferred developers for this invention comprise aqueous solutions of inorganic substances which are reactive towards the aluminum surface and which at the same time produce a water-receptive protective film. These include soluble phosphates, silicates and fiuoborates. A variety of synthetic organic detergents may be employed as detailed herein including mixtures of wetting agents. A major proportion of anionic detergent is preferred. For instance, one preferred composition contains the ammonium salt of a sulfated dodecylphenoxypoly(ethyleneoxy)- ethanol and a sodium alkyl arylsulfonate.

In the prior art, alkaline conditions are represented as effective for the development of diazo oxide containing layers. With the sensitizing layers and surface of this invention, this teaching is not applicable. I find, instead, that the developability of my layers is profoundly affected by the nature of the aluminum surface supporting the sensitizing layer and no general rule can be given to the utility of simple alkalis.

lElate A of Example 1, which is my preferred process, could not be satisfactorily developed with any of the simple alkalies, film-forming developers alone yielding clean backgrounds.

EXAMPLE I Sheets 29 /2 x 38 /2 and 0.020 inch thick of type 3003 aluminum were degreased by passing the sheet slowly through a spray washing machine containing sodium carbonate cleaner at a concentration of 3% (4 oz./ gal.) and a temperature of 75 C. for a total contact time of 0.25 minutes and then washed free of detergent by a spray of cold water and subsequently dried by infrared heat.

Following the degreasing step, the sheet was mounted on a mechanical graining machine similar to that described in U.S. Patent 2,495,269 and heavily grained with air fluidized aluminum oxide abrasive at a speed of about 2500 square inches grained per minute. Viewed under a microscope at 400x, the surface was completely roughened with pores of an apparent average size of about 2 millimeters wide and 3 millimeters deep, the surface abounding in jagged hooks of metal and reentrant pores. Following the graining operation, the heavily curled sheet was dedusted by an air blast to remove superficial abrasive dust and then given two decurling passes through a Pexto decurling machine manufactured by Peck, Stowe and Wilcox, the first pass being parallel to the graining direction and the second normal thereto. A substantially fiat sheet was then passed through another spray washing machine to remove remaining dust and debris deposited during the graining operation. Sodium carbonate was again employed, this time at a concentration of 0.031 lb. per gallon at a temperature of 75 C. and for a total contact time of 0.1 minute. Following the final washing, the plates were treated in the following ways:

(A) One set of plates was dried by infrared heat.

(B) One set of plates was immersed in a hot solution of 5 5% sodium metasilicate for 2 /2 minutes, washed with cold water, and then dried with infrared heat.

A sensitizing composition having the following ingredients was prepared:

Grams Vinylite AYAC l Sensitizer P l0 Cyclohexanone 80 Vinylite AYAC is a low molecular weight polyvinyl acetate resin marketed by Union Carbide Plastics Corporation.

Sensitizer P is a diazo oxide sensitizer prepared by reacting 2-diazo-l-napthol-S-sulfonyl chloride with R0- sin-Amine D, a product of the Hercules Powder Co., Wilmington, Del. Sensitizer P and its preparation are fully described in Example I of U.S. Patent 2,797,213.

A pool of 15 cc. of the above sensitizing composition was poured onto a sample of each of the plates above and rapidly spread over the plate surface using a cheesecloth pad weighing about 25 grams. Two complete coverages of the plate were made with this first pad, once across the plate and once normal thereto. The first pad was then discarded and a second pad used to rapidly wipe over the plate to remove local excesses of sensitizer composition. Two coverages were made with the sec-0nd pad. The second pad was then discarded and a third pad used to buff down and polish the coating to drynes. A smooth polished highly uniform sensitizing layer resulted. The plate was then ready for exposure without further drying.

The plate coated as described was exposed in a vacuum frame beneath a positive original for one minute to the light of a 17-ampere carbon arc distant 56 inches from the plate.

Following the exposure, the plate was placed in a sink and flooded with a developer solution having the following composition:

Grams Sodium metasilicate (Detergent) Sorapon SF-78 /2 Water 94 /2 The detergent listed above is the trade name of the Antara Chemical Division of the Chemical Group of G A F Corporation for a material described by this company in a product bulletin as a sodium alkylaryl sulfonate.

The flooded plate was wiped over with a cotton swab. The plate in the course of a few seconds time turned a deep purple blue color which then lifted out and disappeared as the coating composition was peptized and dispersed in the exosed areas by the combined action of the sodium metasilicate and the wetting agent. In order to insure complete development, the plate was flooded a second time with the developer composition and swabbed over. The total of development was about 45 seconds although contact with the developer could have been prolonged indefinitely without harmful results. Following development the plate was flushed front and back with water and gone over with a 5 Baum gum arabic solution in water. While still wet the plate was treated with a letterpress type of ink diluted one to one with cyclohexane solvent. This was for the purpose of imparting sufficient color to the image areas so that staging operations could be carried out on the plate, if required. Following the inking operation, the plate was Washed with water and treated with a 7 Baum gum arabic solution which was rubbed to dryness. The plate was then ready for storage prior to mounting the completed plate on the offset press. A plate according to recipe (A) above of identical size made in a commercial plant in an exactly similar manner to that just described yielded one hundred twenty-six thousand impressions before any wear could be detected. A plate made according to recipe (B) failed after one hundred impressions. Both plates showed exceptionally well desensitized nonimage areas.

6 EXAMPLE 11 Sheets 10 inches x 16 inches and 0.006 inch thick type 3003 aluminum were degreased by using a 20% trisodium phosphate solution at a temperature of 71 C. for 5 minutes. Considerable etching or chemical graining of the aluminum surface took place during the degreasing step and a black smut was formed on the surface of the sheet. Use was made of a well established procedure for the removal of aluminum cleaning smut by treating the plates for 2 /2 minutes in cold concentrated (70%) nitric acid. Afterwards the plates were washed with cold water and dried by infrared heat. Sheets so treated were grained as described in Example I. Following the graining operation, the plates were dedusted and decurled as already described in Example I. Following the decurling operation the plates were treated in the following different ways:

(C) One set of plates was washed with hot water containing a trace of detergent to remove graining debris and dried with infrared heat.

(D) One set of plates was treated after graining exactly as in Example I(A).

(E) One set of plates was treated after graining as in (C) above of this example except that after the washing step the plates were immersed in a solution of 5% sodium metasilicate for 5 minutes, washed free from silicate and dried with infrared heat.

(F) One set of plates was treated after graining as in (D) above of this example except that following the aftergraining sodium carbonate wash, the plates were silicated as in (E) above.

Samples of plates made according to each of the several procedures outlined above were sensitized using the sensitizing composition and methods outlined in Example I. Following exposures to light the plates were processed as outlined in Example I. Plates (C) and (D) appeared normal showing adequate image strength, adequate ink receptivity and well desensitized backgrounds. Plates (E) and (F), however, exhibited local blindness. On press testing, plates (E) and (F) failed at 20 impressions while plates (C) and (D) were in excellent condition at 10,000 impressions at which point the run was terminated.

Plates (E) and (F) of this experiment and plate (B) of Example I show the advantage of having an active aluminum surface on the plate.

EXAMPLE III Plates were prepared, sensitized and exposed according to the procedures given in Example I. Following exposure, the plates were developed in a solution of the following composition:

Water to grams.

Development of the plate proceeded as in Example I. Following development, the plate was treated with 7 B. gum arabic and thereafter with a letterpress ink diluted with its own volume of high boiling petroleum ether. Excess ink was removed by more 7 B. gum arabic and the plate washed off with water. The image area was very greasy and the nonimage areas very water receptive. Control plates omitting the phosphate were ink receptive overall after processing. Other plates were developed with sodium hydroxide and the wetting agent of Example I. This yielded an overall ink-receptive condition. Finishing the control plates with phosphoric acid and phosphoric acid plus gum failed to improve the ink-receptive condition in the nonimage areas. On press testing the plates having silica beneath the image failed after a few impressions.

7 EXAMPLE IV In place of the trisodium phosphate of Example III a developer of the following composition was used:

Grams Sodium sesquisilicate Sorapon SF-78 /2 Water to 100 grams.

and the procedures of Example III were followed.

The test plates with sodium sesquisilicate showed excellent development, an image of good water and gum repellency, and extremely well desensitized nonimage areas. However, the presilicated plates failed on press testing after a few impressions.

EXAMPLE V The Procedures of Example IV were followed except that in place of sodium sesquisilicate, sodium orthosilicate was employed. Comparable results were obtained.

EXAMPLE VI The procedures of Example IV were followed except that in place of sodium sesquisilicate, a solution of sodium fluoroborate buffered to 11.8 by means of sodium hydroxide and containing /2 gr. of Sorapon was employed. Excellent results were obtained in the active surface plates. Plates otherwise identical except that fluoborate was omitted showed ink sensitivity to a high degree in the nonimage areas. Similar experiments were carried out in which sodium fluoride in one case and sodium borate in another case were used in place of the fluoborate. Both of these materials were ineffective.

EXAMPLE VI I In place of the AYAC resin of Example I, 10 grams of Acryloid C-10, a methacrylate polymer manufactured by Rohm & Haas Co., Philadelphia, was used and plates coated and processed as described in Example I. Substantially equivalent results were obtained. Other plates were processed using the developer compositions of Examples III, IV, V and VI with the results similar to those in the examples mentioned.

It should be noted that the materials mentioned can be varied through wide limits without encountering inoperability. Thus, potassium silicate is as equally useful as sodium silicate. On the other hand, calcium silicates are somewhat too insoluble to be useful. A variety of synthetic organic wetting agents can be employed although anionic types in major quantities are preferred. Thus, in any of the foregoing equivalent quantities of Benax-2A1 (Dow Chemical Company, Midland, Mich.), described by the manufacturer as a disodium-4-dodecylated oxybenzenesulfonate, or Nacconal-S2 (National Aniline Division of Allied Chemical Corporation, New York, N.Y.), described by the manufacturer as an alkyl aryl sodium sulfonate can be substituted for the Sorapon SF78. Neutronyx 640 (Onyx Chemical Corp., 190 Warren St., Jersey City 2, N1), described as a nonionic polyoxyalkalene condensate can be employed, preferably in admixture with a major quantity of an anionic type. The concentration limits of the sensitizing compositions are equally broad although we prefer to keep the total solids at less than 30% while less than 5% is prone to put down too thin a coating with low viscosity resins. For hand coating the upper limit is set by the viscosity of the sensitizing layer as it is applied to the plate, too high a viscosity making it impossible to obtain a smooth, thin, highly uniform coating. This limitation does not apply to presensitized plates.

The composition of the developer is not critical with respect to concentration, Less than 1% wetting agents is likely to be rather ineffective while more than 1% is usually wasteful. Similarly, the reactive material concentration may vary according to solubility up to or higher as in the case of the alkali silicates.

Among the principal differences between the plates of this invention and those of the prior art are the following:

(1) The sensitizer is coated on active mechanically grained aluminum surfaces which at least in combination with the simple alkaline developers of the prior art yield a nonimage condition not readily desensitized except by the developer here employed.

(2) The compositions of this invention are adapted to coating immediately before plate exposure and processing, permitting the use of a sharp, full grain not practical with presensitized plates of the prior art. This avoids shipping difiiculty, size limitation, thickness limitation, plate curl problem and shelf life problem.

(3) This invention yields nonprinting areas having better desensitization than the nonprinting areas of the prior art and at the same time much superior image adhesion.

(4) This invention permits use of substantial proportions of alkali-resistant resins of the deep-etch lacquer type in place of the soluble additives of the prior art whose proportions in the layer are not commensurate with those of the sensitizers. In the preferred composition, the ratio of resins to sensitizers is, for instance, 6 to 4. Superior run length and faster rinting speeds are obtained.

In the claims which follow the specific diazo oxide is referred to as N-dehydroabietyl[6-diazo-5(6)-oxo-1-napthalene sulfonamide] in accordance with the nomenclature of US. Patent 2,797,213 (Example 1).

It is to be understood that although the specific embodiments described above refer to wipe-on plates, this invention is also useful with presensitized plates.

I claim:

1. A plate capable of conversion to an offset printing plate comprising a sheet of aluminum of which at least one surface is mechanically grained and selected from the class consisting of aluminum and the aluminum oxides formed by exposure to air, said surface being coated with a sensitizing layer comprising essentially a mixture of alkali-resistant, water-insoluble, solvent-soluble adhesive polyvinyl resin selected from the group consisting of polyvinyl acetate and polymethacrylate resins, and a lightsensitive water-insoluble diazo oxide of an aromatic sulfonamide in which the =N and =0 always occupy an adjacent position in the same ring and in which the amido nitrogen is substituted by an alicyclic radical of high molecular weight.

2. The product as defined in claim 1, wherein the adhesive resin is polyvinyl acetate.

3. The product as defined in claim 1, wherein the adhesive resin is a polymethacrylate.

4. The product as defined in claim 1, wherein said diazo oxide is N-dehydroabietyl[6-diazo-5(6)-oxo-1-napthalene sulfonamide] 5. The product as defined in claim 4, wherein the adhesive resin polyvinyl acetate.

6. The product as defined in claim 4, wherein the adhesive resin is a polymethacrylate.

7. The process of producing a lithographic plate having a positive printing image which comprises coating at least one surface of a sheet of aluminum, said surface being mechanically grained and selected from the class consisting of aluminum and the aluminum oxides formed by exposure to air, with a sensitizing layer comprising essentially a mixture of alkali-resistant, water-insoluble, solventsoluble adhesive polyvinyl resin selected from the group consisting of polyvinyl acetate and polymethacrylate resins and a light-sensitive water-insoluble diazo oxide of an aromatic sulfonamide in which the =N and =0 always occupy adjacent positions in the same ring and in which the amido nitrogen is substituted by an alicyclic radical of high molecular weight, exposing said coated plate to a positive original and developing with an aqueous alkaline solution having dissolved therein a synthetic organic detergent and a material which is reactive in an alkaline solution with aluminum to produce a hydrophilic film.

8. The process of claim 7, wherein the adhesive resin is polyvinyl acetate.

9. The process of claim 7, wherein the adhesive resin is a polymethacrylate.

10. The process of claim 7, wherein the diazo oxide is N-dehydroabietyl[6-diazo-5(6)-oxo-1 napthalene sulfonamide].

11. The process of claim 7, wherein said material is selected from the group consisting of water-soluble silicates, finoborates and phosphates.

12. The process of claim 7, wherein said material is a water-soluble silicate.

13. The process of claim 7, wherein said material is a water-soluble fluoborate.

14. The process of claim 7, wherein said material is a water-soluble phosphate.

15. The process of claim 7, wherein said detergent comprises a major proportion of an anionic detergent.

References Cited UNITED STATES PATENTS 3,110,596 11/1963 Heiss et a1 96-33 XR 3,128,181 4/1964 Doggett 96-33 3,149,972 10/ 1964 Herrick et a1. 9649 XR GEORGE F. LESMES, Primary Examiner M. WITTENBERG, Assistant Examiner US. Cl. X.R. 

